Malayaite

Malayaite
General
Category	Silicate mineral
Chemical formula	CaSnO[SiO4]
Crystal symmetry	Monoclinic prismatic (2/m)
Unit cell	a = 7.15 Å, b = 8.90 Å, c = 6.67 Å; β = 113.4°; V = 389.50 Å3; Z = 4
Identification
Molar mass	266.87 g
Color	Colorless, greenish gray, white, light yellow, orange
Crystal habit	Massive or wedge shaped
Crystal system	Monoclinic
Mohs scale hardness	3.5 - 4
Luster	Vitreous or resinuous
Streak	White
Diaphaneity	Translucent
Specific gravity	4.3 - 4.55
Optical properties	Biaxial (-)
Refractive index	nα = 1.765 nβ = 1.784 nγ = 1.799
Birefringence	δ=0.0340-0.0350
Pleochroism	x=colorless, y=colorless, z = pale yellow
Other characteristics	Fluorescent, Short UV = bright yellowish green
References	[1][2][3][4]

Malayaite is a calcium tin silicate mineral with formula CaSnO[SiO₄].^[2] It is a member of the titanite group.

Discovery

Malayaite was originally found in Perak (a state in Malaysia) and was first described in literature in 1961, though it was not yet given a name.^[5] In 1965, the mineral was named and recognized by the International Mineralogical Association. It was named for the locality in which it was discovered; which is the Malay Peninsula in Malaysia.^[1]

Crystal structure and symmetry

This mineral is classified under the nesosilicate group for silicate minerals because the titanite group falls under this category as well. Minerals in the nesosilicate group have isolated SiO₄ tetrahedra connected to cations. In malayaite, the tetrahedra are connected to the chain of distorted SnO₆ octahedra, in which the octahedra are linked by vertex [trans corners] sharing and form chains parallel to the miller index of [100]. Within the SnO₆-SiO₄ framework, the CaO₇ polyhedra form chains parallel to [101].^[6]

Malayaite belongs to the monoclinic crystal system and has a 2/m (prismatic) crystal class. According to the Hermann–Mauguin notation, the '2' refers to the two-fold axis while the 'm' refers to the presence of a single mirror plane. The '/' symbol indicates that the two fold axis is perpendicular to the mirror plane.

The space group for malayaite is A2/a. According to the Bravais lattice symbol, 'A' refers to single face centering of the motif. This means that there is one more point on the center of one face. '2' refers to the two-fold rotation axis while the '/' indicates that a mirror plane is perpendicular to the a-axis, which is the last symbol on that space group notation.

Optical properties

Malayaite exhibits anisotropy so it is doubly refractive and breaks up the light that travels through it into two distinct rays of different speeds^[7]. This mineral is known to have very high optical relief and has three indices of refraction. Since it is a monoclinic mineral, Malayaite exhibits two different colors under plane polarized light making it a pleochroic mineral. Malayaite is biaxial birefringent (trirefringent).

Functions and purposes

Malayaite has excellent thermal properties and stability making its host lattice ideal for ceramic pigmentation unlike its analogous mineral, titanite that undergoes phase transition at a higher temperature.^[8] This makes chromium-doped malayaite very desirable as it provides a consistent pigment for ceramics. Malayaite contains the element tin which is sometimes doped with chromium or nickel. These chromophore elements allows the mineral to produce different pigmentation. If malayaite is chromium-doped, it produces a pink-red color^[9] while with nickel, it produces a purple^[10] color. Pink chromium-doped malayaite Ca(Sn,Cr)SiO₅ is among a few important minerals used in producing pigments for the ceramic industry, specifically for coloring glazes as it produces a resilient color.^[11]

Occurrence

This mineral is generally found in tin-rich contact metamorphic skarn deposits. There is a possibility that malayaite is a hydrothermal altered form of cassiterite-quartz assemblage or some tin-bearing mineral. There have been specimens that were found with malayaite as a coating mineral on cassiterite.

References

1. ^ ^{a b} Mindat
2. ^ ^{a b} WebMineral
3. ^ Handbook of Mineralogy
4. ^ Higgins, J.B., and Ribbe, P.H., (1977) The Structure of Malayaite CaSnOSiO₄, a tin analog of Titanite, American Mineralogist, 62, 801-806.
5. ^ Ingham, F.T., and Bradford, E.F. (1961) First description of malayaite as an unnamed mineral. American Mineralogist, 46, 768.
6. ^ Higgins, J.B., and Ross, F.K. (1977) The crystal structure of Malayaite: CaSnSiO₅. Crystal Structure Communications, 6, 179–182.
7. ^ Gemology Project Retrieved 2010-10-20
8. ^ Heyns, A.M., and Harden, P.M. (1999) Evidence for the existence of Cr(IV) in chromium-doped malayaite Cr⁴⁺; CaSnOSiO₄: a resonance Raman Study. Journal of Physics and Chemistry of Solids, 2, 2, 277-284.
9. ^ Lee, H.S, and Lee B.H (2008) The development of a Chromium pink glaze coloring. Journal of Ceramic Processing Research, 9, 3, 286-291.
10. ^ Halefoglu, Y.Z., and Kusvuran, E. (2010) Preparation of ceramic pigments by sol-gel and combustion methods. Journal of Ceramic Processing Research, 11, 1, 92-95.
11. ^ Costa, G., Ribeiro, M.J., Labrincha, J.A., Dondi, M., Matteucci, F and Crucian, G. (2008) Malayaite ceramic pigments prepared with galvanic sludge. Dyes and Pigments, 78, 2, 157-164.